The sample was right. The first production lot was mostly right. By the third lot something has drifted — a colour, a dimension, a label position — and nobody at the factory seems to know when it changed. This is the most common shape of a quality problem in contract manufacturing, in Vietnam as anywhere, and it is rarely about skill. It is about what happens to a specification between the meeting room and the production line, and about changes nobody told you about.
Pre-shipment inspection catches some of this at the end. This article is about the controls that stop it earlier: how the specification reaches the operator, how changes are reported, what is checked on the line, and how a corrective action is closed. Factory selection, sample approval and inspection sampling each have their own article, linked where they come up.
💬 Production drifting away from the approved sample? Tell us what has changed and we can see the line and the records for you.
Where production problems usually come from
Most quality problems trace to five mechanisms — the specification losing detail on its way to the line, the gap between sample and mass production, unreported changes, undisclosed subcontracting and material lot variation — not to the capability of the people making the part.
| Mechanism | What it looks like | The control that stops it |
|---|---|---|
| Specification lost in transmission | The engineer understood; the line operator never saw the requirement | A one-page bilingual line specification at the workstation |
| Sample versus production gap | Samples made slowly by senior staff; production on another line by other people | Pre-production sample made on the production line |
| Unreported change | A new resin lot, a different machine, a new operator, a “small” process tweak | Written change notification before the change |
| Undisclosed subcontracting | A finishing step quietly moved to another workshop | Process route agreed and frozen; subcontractors named |
| Material lot variation | Same grade, different behaviour | Lot traceability and incoming checks on critical materials |
Notice that none of these is solved by inspecting harder at the end. Each needs a control at the point where the problem enters.
Make the specification reach the line
A specification that lives in an English email and a PDF drawing has not reached the person making the part; translate the critical requirements into a one-page Vietnamese line sheet with photographs, and keep it at the workstation.
Your drawing and specification are read by the factory’s engineer or sales contact. What the operator sees is whatever the factory turns that into. Ask the factory to produce, and show you, the line document for your part. A useful one fits on a page and carries:
- the critical characteristics only — the five or six things that make the part right or wrong;
- photographs of a good part and of each known defect, labelled in Vietnamese;
- the check the operator performs, with which gauge or reference, and how often;
- what to do and whom to call when a check fails.
If you can read only the English half, have someone read the Vietnamese half for you. Translation is where “no visible scratches on face A” becomes “no scratches”, which is a different and unachievable requirement, or “check colour” with no reference to what colour.
Close the gap between sample and production
A sample proves the part can be made; only a sample made on the production line, by production staff, at production speed, proves it will be.
Development samples are often made by the most experienced people, slowly, sometimes on different equipment. Insist that the pre-production sample — the one production will be checked against — comes from the real line, real tooling and real material lot. The stages and how to record each approval are in sample approval stages before a production run. Keep a counter-sample of that approved unit on both sides; it becomes the reference at the workstation and in any later dispute.
💬 We can witness the pre-production run in Vietnam and keep the approved reference on file — see sourcing from Vietnam.
Require notification before any change
The single most effective clause against drift is a written obligation to notify and obtain approval before changing material, machine, method or people on your part.
Factories serving Japanese customers often know this discipline as “4M change” control — man, machine, material, method — and many Vietnamese suppliers to Japanese buyers already file 4M change reports. Whatever it is called, write it into the purchase terms:
- Material: a new grade, a new material supplier, or a substitute for an out-of-stock item.
- Machine: moving the job to another machine, line or tool; tool repair or replacement.
- Method: any change to the process sequence or parameters, packing, or the inspection method; moving a process to a subcontractor.
- People: a new line leader or a new team on a critical process, including after the Lunar New Year holiday.
For each, the factory notifies in writing before the change, and the first lot after the change gets first-article-level inspection. A change made without notice is then a breach you can point to, rather than a surprise you have to diagnose. Material substitution in moulding is covered in detail in injection moulding in Vietnam.
Check during production, not only at the end
An inspection at the end of production can only sort; checks during production can stop the line before a whole lot is made wrong.
| Checkpoint | What is checked | Who records it | What it prevents |
|---|---|---|---|
| Incoming material | Grade, lot number, certificate against order | Factory incoming inspection | Wrong material entering the process |
| First piece of each shift or set-up | Critical dimensions and appearance against the reference | Line leader or factory QC | A wrong set-up running for a whole shift |
| In-process patrol | Critical characteristics at fixed intervals | Factory QC (IPQC) | Gradual drift going unnoticed |
| During-production inspection | A sample of finished units early in the run | You or a third party | Discovering a problem when the lot is complete |
| Pre-shipment inspection | The finished lot against the reference | You or a third party | Shipping a non-conforming lot |
Ask to see the first-piece and patrol records for your part, not only the final inspection report. Records written in ink at the time, with gaps and corrections, are more credible than a perfect form filled in afterwards. The last two rows — during-production and pre-shipment inspection — are set out in pre-shipment inspection and AQL.
Freeze the process route and the subcontractors
Agree in writing which process happens where, name the subcontractors, and require approval before any step moves.
Plating, painting, heat treatment and printing are commonly outsourced in Vietnam. That is normal. What creates quality problems is a step that moves without anyone telling you, so the parts are now treated by a workshop whose process nobody approved. List the route at the start, attach it to the order and treat any change as a change under the notification clause. How to ask about subcontracting on a factory visit is in what to check on a first visit to a Vietnamese factory.
Plan for the calendar
The weeks around the Lunar New Year combine a rush before the break with staff changes after it, so plan inspection, stock and approvals around them.
Production pressure rises before the holiday, and teams can change afterwards; the first lots after the restart deserve the same attention as a first production lot. Schedule a during-production check on the first post-holiday lot and avoid starting a new part in the last weeks before the break. The full calendar is in planning production around Tết.
Close the loop with a written corrective action
A defect is only closed when the cause is identified, the fix is in the process, the line document is updated and the next lot shows the effect.
Ask for a corrective and preventive action report in writing, covering: what happened and how many units; the root cause, separated into material, machine, method and people; the immediate containment; the permanent change; and how its effect will be verified. Then check that the line sheet at the workstation has actually been updated — a corrective action that exists only in a report has not reached the person who makes the part. What to do with the defective lot itself, and who pays, is covered in defects or delays from a Vietnamese factory.
💬 For each project we find, vet and manage the Vietnamese manufacturer, and follow the line between samples and shipment. Tell us about your product.
Frequently asked questions
1. Why was the sample good and the production lot not?
Usually because the sample was made by different people, on different equipment or at a different pace from production. Approve a pre-production sample made on the real line, and keep it as the reference.
2. Will third-party inspection prevent quality problems?
It catches problems in finished goods, which is valuable. It does not correct a specification that never reached the line or a change that was never reported. Combine it with change notification and in-process checks.
3. What should a change notification clause cover?
Changes of material or material supplier, machine or tool, process or packing, inspection method, subcontractor, and the team on critical processes. Require written notice before the change and first-article-level checks on the first lot after it.
4. Should the specification be translated into Vietnamese?
The critical requirements should be, on a one-page line document with photographs. The full specification can stay in English, but the operator needs the parts that decide right from wrong in the language they work in.
5. How do I know a corrective action has worked?
Check the next lots for the same defect, and check that the line document and the process have actually changed. A report on its own is not evidence that anything on the line is different.
Sources: general manufacturing quality practice; ISO 2859-1 for sampling where referenced. Checked on 17 October 2026.